The information in the mass spectrum of a chemical compound is of great value to chemists in identifying the compound and in characterizing its molecular structure. For those compounds for which mass spectra can be obtained, the mass spectrum typically reveals the molecular weight of the compound and the masses of a characteristic assortment of ionic fragments derived from the compound. To generate mass spectra, conventional mass spectrometers operate on a beam of ions derived from the material to be analyzed, either by deflecting the beam electromagnetically in a way which depends upon the ratio of the mass to the charge of the ions in the beam, or by measuring transit times of ions in a pulsed ion beam. Compounds which are to be mass analyzed generally must be converted into an ion vapor before introduction into the section of a conventional mass spectrometer which forms the ion beam.
The need to convert materials to be mass analyzed into an ion vapor has been a source of problems in the field of mass spectrometry for which no completely satisfactory solution has heretofore existed. Gaseous compounds or compounds which can be thermally vaporized without decomposition can usually be converted to an ion vapor relatively easily by heating the compound to vaporize it if it is not a gas, and either bombarding the compound in the gaseous state with a beam of electrons (electron impact ionization) or by introducing chemically-reactive ions into the gas (chemical ionization). However, many compounds are not sufficiently volatile at ambient temperatures to form a gas suitable for either electron-impact ionization or chemical ionization, and moreover, decompose when heated so that they cannot be vaporized thermally. Among the compounds which cannot be converted into an ion vapor by these conventional techniques are many which are of biological, medical and pharmaceutical interest.
A number of special techniques have been developed to generate an ion vapor from compounds of low volatility. These techniques include field desprotion, laser-assisted field desorption, plasma desorption, rapid evaporation from inert surfaces, and secondary ionization mass spectrometry. Literature citations to these and other techniques may be found in Analytical Chemistry, vol 52, pp. 1589A-1606 (Dec. 1980). None of these techniques is without its limitations, however, and a need still exists for improved methods for obtaining mass spectra of involatile, heat-sensitive materials. The problems of forming an ion vapor of involatile and heat-sensitive compounds become particularly acute when it is attempted to use a mass spectrometer to analyze the effluent of a liquid chromotograph. Liquid chromotographs are widely used to separate mixtures into their component compounds, and find particular application when one or more of the component compounds is too involatile to permit the mixture to be separated with a conventional gas chromotograph. Although mass spectrometers have been widely and successfully interfaced to gas chromatographs to permit mass spectra to be taken of compounds in the gaseous effluent from the chromatograph, efforts to interface liquid chromatographs to mass spectrometers have been less successful, in part because compounds eluted from the liquid chromatograph are frequently involatile and heat sensitive and thus not amenable to conversion into an ion vapor by conventional techniques. Moreover, the compounds to be analyzed from the liquid chromotograph are dissolved in a volatile solvent, which tends to reduce the ionization efficiency of the mass spectrometer even further with respect to the solute compounds of interest since solvent vapor is generally ionized along with the solute compounds and the solvent is typically in a much greater concentration than the solute compounds.
During the past decade several laboratories have worked on the development of combined liquid chromatograph-mass spectrometer systems. Much of this work has focused on dealing with the fact that the mass flows involved in conventional high pressure liquid chromatography (ca. 1 g/min) are two or three orders of magnitude larger than can be accommodated by conventional mass spectrometer vacuum systems. The status of various approaches to overcoming the apparent incompatibility between liquid chromatography and mass spectrometry has recently been summarized by P. J. Arpino in Trends in Anal. Chem. 1, 154 (1982) and Biomed. Mass Spectrum 9 176 (1982).
An early approach to LC-MS employed laser heating to rapidly vaporize both the solvent and sample and molecular beam techniques to transport and ionize the sample while minimizing contact of the sample molecules with solid surfaces. This apparatus used a large and expensive vacuum system and a 50 watt CO.sub.2 laser. Later, a greatly simplified version on this system used oxy-hydrogen flames to vaporize the LC effluent and a substantially less elaborate vacuum system. This system is described in an article by C. R. Blakely, J. J. Carmody and M. L. Vestal in Anal. Chem. 52 1636 (1980).
Earlier systems were designed to efficiently transfer the sample to either an electron impact (EI) or chemical ionization (CI) source while vaporizing and removing most of the solvent. In the latter version more than 50% of the sample was transferred to the CI ion source with only about 5% of the solvent vapor. This system gave satisfactory performance for a number of relatively nonvolatile samples and was used successfully with reversed phase separations employing aqueous buffers at flow rates as high as 1 ml/min. The major problem with this system was that the vaporizer was difficult to control properly which often caused uncontrolled fluctuations in performance and some times frustrated attempts at application of the system to real analytical problems.
In the course of this work it was found that, under certain conditions, ions were produced even though the hot filament normally used to produce the primary ionizing beam was turned off. Initial measurements of mass spectra produced from nonvolatile compounds such as peptides, nucleosides, and nucleotides, showed that the spectra were quite different from those obtained by chemical ionization and were, in fact, most similar to those obtained by field desorption.